Simulation Study On Ammonia Storage Dynamic Characteristics Of SCR Catalyst On Heavy-duty Diesel Engine

Nitrogen oxide emissions contribute significantly to photochemical smog and also toacid rain.Therefore many attempts are currently being made to further reduce theseemissions especially from large diesel engines used for non-stationary applications.In therecent years, considerable progress has been achieved in the development of urea-SCR fornonstationary, mobile applications. Main challenges are the reduction of the catalyst volumesand the optimization of the dosing strategy to minimize transient ammonia emissions duringload changes.In order to further characterize and optimize the performance of Selective CatalyticReduction aftertreatment systems used on heavy duty diesel engines, an accurately calibratedglobal kinetic SCR model is desirable. The model parameters were calibrated using anoptimization code to minimize the error between measured and simulated NOx gasconcentration time histories. The calibrated SCR model is able to predict the effect oftemperature and velocity on SCR performance and is capable of predicting gaseous NOx,and NH3storage on the catalyst. In this study, a V2O5-WO3/TiO2SCR catalyst from a12Ldiesel engine was experimentally studied in an enginge bench test using carefully designedprotocols.With the increase of the ammonia concentration, the NOx conversion rate increases.However, the high ammonia concentration is easy to form ammonia slip at lowtemperature.Under a certain temperature range, NOx conversion rate increases with theincrease of temperature. SCR catalyst has a high catalytic activity at high temperature, whereis the SCR catalyst ideal working range, but higher temperature will lead to ammoniaoxidation and reduce NOx conversion.The change of the exhaust flow has little influence onNOx conversion, while NOx conversion rate affected by the exhaust flow rate is obvious atlow temperature,where he catalyst activity is relatively low.The ammonia storage capacity of SCR catalyst carrier is studied under the change of temperature and exhaust flow. Ammonia has an epsilon shape distribution in SCR catalyst,and the catalytic surface coverage is degressive along the axial direction. the radialdistribution of Ammonia is concentric circles, due to the dual effects of temperature andvelocity.With the increase of temperature, the catalytic surface coverage of ammoniadecreased,and the moment of ammonia slip is delayed.The effective SCR zone becomesmaller, mainly because of higher catalytic activity at high temperature, NOx is fully reactedin front of catalyst. With the increase of the exhaust flow, the ammonia storage capacityincreases, and ammonia slip appear earlier. The ammonia storage capacity is the same stageat the slip moment. So the slip point is not affected by the exhaust flow. Exhaust flow ratehas no effect on the axial distribution, only the adsorption magnitude of the axial position;The exhaust flow had a greater influence on radial distributionof ammonia.